Coated photoflash lamp



April 11, 1961 v. CHIOLA ETAL 2,978,890

COATED PHOTOFLASH LAMP Filed Jan. 28, 1958 INVENTORS. VINCENT CHIOLA and WILLIAM C. FINK BY A4441,

ATTORNEY lamps.

Paten ed Apr. 11,1961.

2,978,890 COATED PHOTOFLASH LAMP Vincent Chiola, Towanda, and William C. FinlqWilliamsport, Pa.,' assignors, by mesneassignments, to Sylvania ElectricProducts Inc'., Wilmington, Deb, a corporation of Delaware Fina Jan. 28, 1958, Ser. No. 711,622

2 Claims. or. 67-31) This invention relates to photoflash lamps and more particularly to photoflash lamps of the type having a glass envelope coated with a transparent protective lacquer,

the envelope enclosing a combustible material and a combustion supporting gas fill as well as a means for igniting the combustible material.

The purpose of providing the glass bulbs or envelopes of these lamps with a lacquer coating has been to strengthen the bulb and to prevent the scattering of the glass andcontents of the bulb should the lamp explode upon ignition. A common practice in flash lamps of larger dimensions, such as those having an interior volume of the order of 30 cubic centimeters, has been to coat the glass bulb on its inner surface as well as its outer surface with an organic lacquer. The inner coating in this case serves to protect the glass envelope and the outer coating from the full intensity of the heat produced on ignition of the combustible contents of the lamp and also adds strength to the bulb structure.

For larger sizes of flash lamps, lacquers having cellulose acetate as the resin base have been found generally satisfactory as exterior coatings. Apparently cellulose-acetate films have the desired strength, flexibility and transparency after, as well as before, the lamps are flashed as long as the external coating is protected from the full impact of the thermal shock produced on flashing by means of an additional coating on the inside surface of the envelope as described above.

Recently there has beena trend toward the use of flash lamps having smaller glass envelopes. Typically, theselamps may have a volume of about 7 cubic centimeters so that the heat given off by the combustible material must be dissipated over a substantially smaller envelope area than in the larger lamps. As a result, the rate of increase of the temperature of the envelopes and the ultimate temperature attained by the glass envelopes is substantially greater than in the larger bulbs.

In the manufacture of lampsof the smaller, or miniature,bulb sizes it is virtually impossible, and at best commercially impractical, to coat the interior of the envelope with a protective lacquer coating. This is primarily for the reason that the opening at the neck of the bulb is so small as to make difficult the admission of a lacquer spray nozzle. Attempts to coat the interior surface of the envelope by filling the bulb with lacquer and thereafter draining the excesslacquer from the bulb also have been generally commercially unsuccessful because of the small diameter of the opening in the bulb. It is therefore seen that makers of miniature types of photoflash lamps have been relegated to the use of only exterior protective lacquersin the face of requirements even more severe than those which are inherent in larger it hasibe'euifound that flash lamps of the smaller size coated only exteriorly with the unmodified cellulose acetate lacquer heretoforel commonly employed for the purpose have been open we serious objection. More particularly, although the lacquer coating even after flash-' 2 ing retains its desirable properties of flexibility, transparency and optimum degree of adherence to the glass envelope over most of the surface of the bulb, it tends to me.t, froth and bubble in certain spots or areas on the surface of the lamp. As is readily appreciated, these spots are left bare and unprotected against possible implosion or explosion of the lamp. It has been recognized that this problem is attributable to unavoidable zones of relatively high temperature in the envelope and various solutions to the problem have been tried or suggested. It has been proposed, for example, to employ two layers of protective organic films applied to the exterior of the glass envelope. Theinner layer, adjacent the outside of the glass envelope is intended to act as a heat resistant layer to protect the outermost film which is desirably of relatively high tensile strength. Unfor-.

tunately the additional steps involved in the application,

drying and inspectionof an additional protective layer of lacquer adds substantially to the cost of the finishedl lamp.

It also has been suggested to employ single lacquer coatings which are more heat resistant than those previously employed. For example, it has been recognized that such coatings may be realized by adding agents to cellulose acetate lacquers which are capable of crosslinking the molecular chains of the cellulose acetate and after application of the lacquer to the glass bulb,

heating the coating to a temperature sufficiently high to cause cross-linking of the cellulose acetate so that curingof the film results. Although cured coatings of these types have been relatively resistant to smoking, charring, melting and frothing upon ignition of the lamps, it has been found that they tend to have an undesirable yellowtinge. Furthermore they have commonly exhibited characteristic brittleness even before ignition of the lamp and, in addition, adhere too securely to the glass surface of the envelope Such adherence is objectionable for the reason that it prevents free movement of the lacquer coating with respect to the glass thus causing concentration of the stresses at any point of failore of the glass envelope and increased failure of the protective lacquer coating along with the glass.

It is, therefore, an object of the present invention to provide a flash lamp having an exterior coating of novel composition adapted to withstand without failure the conditions to which the coating is subjected upon ignition of the lamp.

A further object is to provide a flash lamp having an improved exterior protective coating of the cellulose acetate type. j

A still further object is to provide a flash lamp of the so-called miniature type having no coating on the interior of the glass envelope butan improved cellulose acetate type protective coating on the exterior of the envelope, adapted to withstand the thermal shock to which the envelope is subjected upon ignition of the lamp.

The foregoing objects and other more specific objects of the invention are accomplished through the use of a protective lacquer coating'on the glass envelope of the lamp, which coating contains as its resin base a mixture of cellulose acetate and a melamine derivative of the formula HN-CHgO CHa l CHaOCHrN-O O-N-CHiO OH; H H

trimethylol melamine. The methylol groups are then etherified by reaction of trimethylol melamine with methanol in the presence of an acid catalyst. The resulting melamine product of the general formulaset forth above is a water-solublematerial in the incompletely condensed state. At elevated temperatures ranging from about 100 to 150 C., particularly in the presence of a suitable curing catalyst, it can be condensed to the substantially infusible and insoluble state. The incompletely condensed melamine product can be purchased, for example, as Aerotex M-3 resin, sold by American Cyanamid Comp ts.

An important feature of the invention is that, although the above-described melamine product is capable of cross-linking with the cellulose acetate at elevated temperatures of the order of those mentioned above at which condensation of the melamine product to the infusible state takes place, we do not subject the protective lac-- quer coatings to such temperatures in the manufacture of the lamps of our invention. Instead the lacquer coatings applied to the glass envelopes are. dried at temperatures only sufliciently high and for a period of timesufficient to drive off the volatile solvents in the lacquer. The times and temperatures which we employ in drying the lacquer are not suflicient to cause substantial crosslinking or curing of the resin base.

The dried lacquer coating on the finished flash lamp has all the desirable characteristics which heretofore has made cellulose acetate useful for the purpose. More particularly, it is highly transparent and flexible, and has the desired limited degree of adherence to the glass bulb. However, when the lamp is ignited, it is observed that, unlike the previously known cellulose acetate lacquers, our improved coating does not melt, froth, bubble or pull away from the lamp to leave bare unprotected areas of the glass envelope. Furthermore, after flashing, the coating is still flexible and does not exhibit objectionable adherence to the glass envelope of the lamp.

Although the reasons for the improved characteristics of our novel lacquer coating. are not fully understood, it is believed that the improved heat resistance results from at least a partial curing of the coating (i.e., crosslinking of the cellulose acetate by the melamine product) by the heat generated during flashing of the lamp. Thus the heat generated in flashing, and which would normall-y cause melting of the cellulose acetate, apparently is expended in raising the energy level of the system con taining the melamine compound to the point where rapid cross-linking takes place, with the production of a ma terial of higher melting point than ordinary cellulose acetate.

Further objects and features of our invention will appear from the following description and the accompanying drawing in which:

' Fig. 1 shows a flash lamp of one type to which the present invention is applicable.

Fig. 2 is a sectional view of the bulb taken on the line 22 of Fig. 1, looking in the direction of the arrows.

Referring to the drawing, there is shown a flash lamp comprising a bulb portion 1 and a base 2 sealed thereto. Inside the bulb and sealed at its lower end to the glass envelope 6 is a mount 4 provided with the usual leads and igniting filaments connected across the leads. Surrounding the leads and filament within the bulb is a mass of filamentary combustible flash material 3 such as shredded aluminum or Zirconium randomly oriented. The envelope 6 is coated with a protective transparent plastic layer of the composition and applied in the manner hereinabove generally described. The bulb also contains a combustion supporting material such as oxygen for supporting the combustion of the filamentary combustible material.

The protective lacquer coatings employed in accordance with our invention may comprise various known solvents and plasticizers, and the components of the coatin'gs may be incorporated into the composition in various orders of addition. We have found it particularly convenient to first prepare a cellulose acetate lacquer typical of those heretofore employed and then add the melamine product to the lacquer. For example, a cellulose acetate lacquer was prepared having the following composition Parts by weight Cellulose acetate 20.0 Acetone 58.0 Denatured ethyl alcohol 14.0 Diacetone alcohol 7.6

Dibutyl phthalate 0.4

To the above lacquer composition was added 2 percent by weight of the composition of an aqueous solution of the above melamine product. The solution of a melamine product contained percent by. weight of the above described N,N,N tris (methoxy methyllmela mine. The resin layer remaining after evaporation of the readily volatile solvents from a film of the modified lacquer contained a mixture of cellulose acetate and the melamine product in a ratio of about 8 parts of the melamine product to parts of cellulose acetate.

A coating of the above-described modified cellulose acetate lacquer was applied to flash bulbs of the type shown in Fig. 1. The volume of these bulbs was about 7 cubic centimeters. The bulbs were dipped, base up, into the lacquer and were then upended and passed, base end down, through a drying tunnel to dry the lacquer film by evaporation of the solvents. About one-half hour was required for movement of the bulbs through the tunnel. The maximum temperature within the tunnel was about 80 C.

The dried lacquer coating on the finished bulbs was from about 3 mils to 4 mils in thickness and was clear and free of any noticeable color. Although the film was drawn tightly around the bulb and there were no indications of bubbles or absence of contact in any areas, between the film and the glass envelope, it was possible without breaking the film to peel an area of the film from the envelope when the peeling was started from a cut through the film. The coating thus exhibited a desired degree of flexibility and limited adherence to the glass envelope.

When the flash bulbs coated as described above were ignited, the lacquer coatings remained intact and did not melt, froth, bubble or pullaway from the glass envelopes' In some cases light brown, charred spots or areas appeared in the coatings after flashing of the bulb, but in all cases, the film could be cut and peeled from the glass without breaking.

In order to determine the effectiveness of the incorporation ofthe melamine product in the cellulose acetate coating, a number of bulbs were coated in the manner described just above, but employing the specific cellulose acetate lacquer of the above composition without the addition of the melamine product. Before ignition of the flash bulbs, the coating appeared to have properties identical with'those of the modified coating. However, upon flashing of the bulbs it was observed that the coating was extensively melted by the heat generated. The coating frothed and bubbled and pulled away from the glass in spots leaving exposed areas of the glass envelope. It was evident from the comparison of the two sets of results that, although prior to flashing the lamps the two types of coatings appeared to be similar, the modified coating, after being exposed to the flash temperature, exhibited characteristics of a'higher melting and'more heat resistant material. Presumably this is attributable to the cross-linking of the cellulose acetate in the com position by the small quantity of melamine product present, as has been mentioned previously in this application.

Various solvents and mixtures of solvents which heretofore have been employed in ordinary cellulose acetate flash lamp coatings may be used in the present modified coatings. Although a mixture of alcohols and ketones were employed in the above specific example, other known cellulose acetate solvents including, for example, chlorinated hydrocarbons and ester type compounds maybe used. Various plasticizers other than dibutyl phthalate likewise may be employed such as, for example, diethyl phthalate and tricresyl phosphate.

From the standpoint of commercial practicability we have found it preferable to apply the coatings herein described by dipping techniques. However, other methods of application, including those involving spraying of the lacquers likewise can be employed. Regardless of the coating method employed, it is preferable that the final coating be from about 3 to 4 mils in thickness to afiord the necessary protection consistent with the desired degree of transparency. As will be readily-appreciated, however, the advantages arising from the use of the present invention are realized even though the protective films are of greater or less thickness.

In the above specific illustration of our invention, the resin base of the lacquer and the final protective coating contains a mixture of cellulose acetate and the melamine product in a ratio of about 8 parts of the melamine product to 100 parts of the acetate. Although this is a particularly suitable composition, compositions containing as little as 5 parts of the melamine product to 100 parts of cellulose acetate may be employed. Lesser quantities of the melamine product do not appear to afford the improvement in heat resistance of the final coating which is desired. Up to 15 parts of the melamine product per Previously in this specification it has been indicated that the coatings which we employ are dried on the flash bulbs at temperatures sufliciently low so that no appreciable cross-linking of the cellulose acetate by the melamine product takes place. The particular temperatures and time employed in drying the films after application of the lacquer to the glass depends primarily on the particular solvents in the lacquer. However, temperatures in excess of 100 C. are to be avoided. We prefer to accomplish the drying step by heating the coated bulbs to a temperature within the range of to C. over a period of 30 minutes to about 1 hour.

We claim:

1. A photoflash lamp comprising a sealed glass envelope enclosing a combustible flash material, a combustion supporting material and means for igniting the flash material, said envelope having on its outer surface a protective lacquer coating comprising the partially cured residue of drying at temperatures not exceeding 100 C. a film of a solution containing cellulose acetate and N,N,N" tris (methoxy methyl) melamine.

2. A photoflash lamp comprising a sealed glass envelope enclosing a combustible flash material, a combustion supporting material and means for igniting said flash material, said envelope having on its outer surface a protective lacquer coating comprising the product of heating to a temperature of between 60 C. and 100 C. over a period of 30 minutes to 1 hour a film of a solution containing cellulose acetate and N,N,N" tris (methoxy methyl) melamine in proportion of 100 parts of cellulose acetate to from 5 to 15 parts of N,N',N" tris (methoxy methyl) melamine.

References Cited in the file of this patent Cohen et al. Oct. 22, 

1. A PHOTOFLASH LAMP COMPRISING A SEALED GLASS ENVELOPE ENCLOSING A COMBUSTIBLE FLASH MATERIAL, A COMBUSTION SUPPORTING MATERIAL AND MEANS FOR IGNITING THE FLASH MATERIAL, SAID ENVELOPE HAVING ON ITS OUTER SURFACE A PROTECTIVE LACQUER COATING COMPRISING THE PARTIALLY CURED RESIDUE OF DRYING AT TEMPERATURES NOT EXCEEDING 100*C. A FILM OF A SOLUTION CONTAINING CELLULOSE ACETATE AND N,N'',N" TRIS (METHOXY METHYL) MELAMINE. 